1. Field of the Invention
The invention relates to the field of power supplies, particularly power supplies used in nuclear magnetic resonance scanners.
2. Prior Art
In medical nuclear magnetic resonance (NMR) scanners a patient is placed within a relatively high magnetic field (e.g., 3 k gauss). The hydrogen atoms of the body (or other specimens) are excited by a radio-frequency signal and then the radio-frequency signals emitted from the hydrogen atoms during the relaxation times of the atoms (T.sub.1 and T.sub.2) are sensed. It is known that the frequency of the emitted radio-frequency signals from the hydrogen atoms are a function of the magnetic field strength.
The magnetic field in these scanners is often made to uniformly increase or decrease in known directions during the time that the hydrogen atoms are excited by the radio signals. The frequency of the sensed radio signals then becomes a function of the magnetic field strength and hence, can be used to determine distance in the direction of the magnetic field gradient.
In one experimental NMR scanner, a cylindrical main coil is submerged in liquid helium. The current induced in this coil provides the uniform magnetic field (e.g., 3 k gauss). The patient is slid into a cylindrical member formed within the central region of the coil and therefore is disposed within the strong magnetic field. Three pairs of other coils (in air) are mounted on the vessel containing the main coil. The axes of the three coil-pairs are generally orthagonal to the main coil. By way of example, each coil-pair has an inductance of approximately 250 microhenries and a resistance of 0.01 ohms per end. With 300 amps passing through these coils, a uniform magnetic gradient is produced within the cylindrical opening of the scanner (e.g., 1 gauss per cm). The direction of this gradient can be changed by exciting different combinations of the coils.
For a general discussion of NMR medical imaging, NMR scanners and the sequencing of the currents through the gradient producing coils, see Nuclear Magnetic Reasonance Imaging Medicine, published by Igaku-Shoin, copyrighted 1981 and edited by Kauffman, Crooks and Margulis.
For some NMR imaging, the current through the gradient coils must be driven quickly to a high level (e.g., 300 amps in 250-300 microseconds), maintained at a steady state for a short period of time and then driven to zero or in the opposite direction to reverse the gradient. Moreover, the current through the coil pairs must be the same, otherwise the magnetic gradient will not be constant and the resultant image will appear to be flattened or expanded in some regions.
In a prior art gradient power supply, a plurality of bipolar transistors are coupled in parallel to control the flow of current through the gradient producing coils. These transistors are in saturation when conducting. When the transistors are turned off, the last one of the transistors to turn off sometimes receives the entire current, that is, the current intended for all the parallel transistors. When this occurs, the transistor can be destroyed. This and other shortcomings to the prior art power supply has led to the development of the power supply described in this application.